For energy saving of power electronics devices such as an inverter, it is necessary to reduce a loss in semiconductor switching elements such as an insulated gate bipolar transistor (IGBT) and a metal-oxide-semiconductor field-effect transistor (MOSFET).
The loss in a semiconductor switching element is determined by a conduction loss or a switching loss of the element. Therefore, in order to reduce these losses, development has been promoted in which wide band gap semiconductors such as silicon carbide (SiC) and gallium nitride (GaN) are used as materials.
With use of a power MOSFET as a switching element, a reflux current can be flowed in a parasitic diode (hereinafter, referred to as a body diode in the present specification) of the power MOSFET. The use of the body diode enables compactification or omission of a reflux diode disposed in parallel with the switching element, and therefore, this technique has been applied to power converter circuits.
There is, however, a problem that bipolar operation of a SiC semiconductor element with use of p-type and n-type carriers expands a crystal defect by recombination energy of the carriers to increase resistance. This problem is generated also in the above-mentioned case of flowing a reflux current in the body diode, leading to problems such as an increase in the loss and malfunction that are caused by an increase in ON-resistance.
In order to solve this problem, Patent Document 1 discloses a structure in which a gate voltage is applied to a channel portion to flow a reflux current during reflux in a SiC-MOSFET to suppress the defect expansion caused by the bipolar operation, so that the increase in the ON-resistance is suppressed. According to this semiconductor device, an increase of the gate voltage to 0 V or more and a threshold voltage or less allows a flow of a reflux current through the channel of the SiC-MOSFET, so that a current can be refluxed with a voltage smaller than necessary for activating the body diode to suppress the bipolar operation.